Cellular telecommunications is one of the fastest growing and most demanding telecommunication applications ever. Today it represents a large and continuously increasing percentage of all new telephone subscriptions around the world.
In Europe and in North America, there are several large analog cellular systems operating such as NMT (Nordic Mobile Telephone) in the Nordic countries, TACS (Total Access Communication System) in the United Kingdom, and AMPS (Advanced Mobile Phone System) in the United States. Quality, capacity and area of coverage vary widely, but demand has outstripped estimates almost universally. To address the capacity limitations of the existing analog cellular systems, many operators are migrating to digital cellular systems. Digital cellular systems are generally classified as either TDMA (Time Division Multiple Access), CDMA (Code Division Multiple Access), or hybrids thereof. TDMA systems, such as the pan-European GSM (Groupe Special Mobile, or Global System for Mobile communication) system and D-AMPS (Digital Advanced Mobile Phone System) in the United States, provide increased capacity by dividing each frequency band into time slots, multiple users being allocated a different timeslot on the same frequency. CDMA systems, on the other hand, provide increased capacity by allowing multiple users to operate simultaneously across the same frequency range through the use of orthogonal spreading codes. In the long-term perspective, cellular systems using some form of digital technology will become the universal way of communication.
TDMA systems, such as GSM and D-AMPS, are currently the most widely deployed digital cellular systems. In GSM, for example, each frequency is divided into 8 timeslots. As shown in FIG. 1, base station 100 broadcasts a signal on frequency F1 to mobile station 120a on timeslot 2 and to mobile station 120b on timeslot 5. Up to eight mobile stations may be accommodated on a single frequency. Signals transmitted from base station 100 to the mobile stations 120a, 120b, are called the downlink signals, or more simply, the downlink. For two-way communication, there must also be a corresponding set of frequencies and timeslots in the opposite direction which are called the uplink signals, or more simply, the uplink. Generally, TDMA systems employ a symmetrical frame format, that is, the TDMA frame structure (i.e., bandwidth, number of timeslots, data rates, etc.) is the same on the uplink as it is on the downlink.
Recently, there have been proposals to construct a mobile satellite communication system employing orbiting satellites in place of, or to supplement coverage from, terrestrial base stations. Generally, mobile satellite communication systems employ a satellite which is in low earth orbit (LEO), intermediate circular orbit (ICO), or geostationary earth orbit (GEO). A ground station serves as the gateway between the satellite and the public switched telephone system (PSTN), or a terrestrial cellular system. Ideally, the satellite functions as a "bent pipe", that is, the satellite is a transponder, relaying signals received from a mobile station down to a ground station and, similarly, relaying signals received from the ground station to the mobile station. Often, a frequency translation in the satellite is required as communications between the ground station and the satellite occur across a different frequency range (e.g. C-band, Ku-band) than do the communications between the satellite and the mobile stations (e.g. L-band). It is generally desirable to design the system such that the satellite has a simple architecture.
TDMA may be used to increase the capacity of communications between the satellite and the mobile stations. As mentioned, TDMA formats are generally symmetrical, that is, the uplink and the downlink have the same TDMA timeslot structure. However, for reasons of performance, the uplink TDMA structure could be different than that of the downlink TDMA structure, that is, the bandwidth, number of timeslots, and data rates may differ significantly between the uplink and downlink.
Where mobile station to mobile station communications are required, passing through the ground station to effect the mapping between asymmetric TDMA formats results in unacceptable time delays. Normally, in an ICO system, the time delay experienced in a connection between the PSTN, or terrestrial cellular network, and a mobile station in communication with the satellite is on the order of 400 milliseconds, which is perceptible to the users, but not quite a hinderance to normal conversation. Where a link between two mobile stations is effected through the ground station, the delay can be as much as 1.2 seconds which results in a unacceptable hinderance to normal conversation.